ADU rules in Hawaii
Representative cities: Honolulu, Hilo, Kailua-Kona.
ADU permit rule
Permit requiredGenerally allowedcounty authority
Fee basis
Valuation-based; set by the county building department (R-3 / U occupancy schedule).
Typical timeline
Plan review typically several weeks; varies by county workload.
Setback
Per county zoning; check shoreline/SMA setbacks on Oahu and other coastal parcels.
Area threshold
Statewide ADU law allows one ADU per single-family lot; counties set size limits (commonly 400–1,000 sq ft by county).
Exempt if
None — an ADU always requires a building permit.
Go / no-go
allowed
Hawaii’s statewide ADU law (Act 50, 2008) allows one accessory dwelling unit per single-family lot; the four counties administer permits and set local size/setback rules. Owner-occupancy and parking standards vary by county.
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Frequently asked questions
Do I need a permit to build an ADU in Hawaii?
Per Hawaii rules, an ADU is "yes" for a permit and the authority is "county". Hawaii’s statewide ADU law (Act 50, 2008) allows one accessory dwelling unit per single-family lot; the four counties administer permits and set local size/setback rules. Owner-occupancy and parking standards vary by county. Always confirm with the issuing building department.
How big can an ADU be in Hawaii?
Area threshold: Statewide ADU law allows one ADU per single-family lot; counties set size limits (commonly 400–1,000 sq ft by county).
What are the ADU setbacks in Hawaii?
Setback: Per county zoning; check shoreline/SMA setbacks on Oahu and other coastal parcels.
How much does an ADU permit cost in Hawaii?
Fee basis: Valuation-based; set by the county building department (R-3 / U occupancy schedule).